1. Field of the Invention
The invention relates to an antenna for reducing an effect of a radio wave blocking obstacle existing in a radio wave transmission path. More specifically, the invention relates to an array antenna having a plurality of element antennas, and to an aperture antenna.
2. Description of the Prior Art
In general, in case of receiving and transmitting radio waves, the reception power is reduced by a blocking obstacle, which exists in the radio wave transmission path between a transmitting point and a receiving point, and blocks the transmission of radio wave.
In order to obtain a desired reception power, it is inevitable to avoid or lessen the reception power decrease.
In the past, for the above requirement, the antennas are arranged at a place where the radio wave blocking obstacle does not disturb the radio wave transmission path.
If the antenna put in the radio wave transmission path is inevitably disturbed by the shadow of the radio wave blocking obstacle due to the change of relative position between the blocking obstacle and the antenna as time passes by, it is well known that the antennas which are placed in different places are switched as time passes by so that two antennas are not caught in the shadow of the blocking obstacle at the same time. Such manner is disclosed in the Japanese Patent Laid-Open publication No. 5-167344 etc., for example. In order to reduce the effect of the radio wave blocking obstacle, the structure of a conventional invention uses one of those antennas which is not caught in the shadow of the blocking obstacle by switching the antennas by turns along the passage of time.
However, although the conventional antennas operate as mentioned above, namely, the antennas which are not caught in the shadow of the blocking obstacle are selected, it is difficult to select which antenna is caught in the shadow of the radio wave blocking obstacle according to the locational relationship between the direction of a satellite, the blocking obstacle and the antennas. In other words, when the radio wave comes in from just above the antenna, the antenna just under the radio wave blocking obstacle is caught in the blocking obstacle's shadow. However, when the radio wave comes in obliquely toward the antenna, the antenna positioned obliquely under the radio wave blocking obstacle is caught in the blocking obstacle's shadow, while the antenna just under the radio wave blocking obstacle is kept away from the shadow. Accordingly, since the decision of which antenna is caught in the shadow of the radio wave blocking obstacle depends on the locational relationship between the direction of the satellite, the blocking obstacle and the antennas, it is difficult to select the antenna which is not caught in the shadow. This imposes restrictions on the arrangement or the structure of antennas and makes the switching control very complicated, in case of a conventional switching type antenna. Especially in case of performing a satellite communication by a rotorcraft like helicopter during the flight, since the rotor acts as a radio wave blocking obstacle, it is extremely difficult to avoid decreasing the reception power.